BB if you need to refresh your diseases/aoe situation, HS otherwise. But the dps difference is pretty small, so you can use BB all the time, with the added bonus that it can't be parried.

its not that low tbh...

On a generic comment:
and its very hard to say: below X targets use this and above X targets use that since the damage both do depend a lot on gear... HS is based on weapon damage and BB not...

The general idea is above 3 targets, you use BB to maximize aggro on all targets since HS only hits 3...
but it doesn't mean that HS cant do more dps only hitting 3 targets than BB hitting 4 targets...

If you are lazy: BB always.
If you are less lazy: HS on bosses, BB on trash and all procs.
If you do it right: Watch your Vengeance and know the coefficients, there are values of AP where BB is more efficient on 1/2/3 target(s) than HS, but I have no idea what they are, and the 1 target one is fairly high, like 100k+.